Signal to Noise Ratio of Monolithic Scintillator Detectors for High Resolution PET

Monolithic scintillator detectors, consisting of several cm³ of scintillating material coupled to one or more Hamamatsu S8550 avalanche photodiode (APD) arrays, have been proposed as detectors for high resolution positron emission tomography (PET). These detectors eliminate the dead space caused by reflective material between crystal pixels, and provide good depth of interaction (DOI) information. The entry point of impinging annihilation photons on the front surface of the detector is derived from the scintillation light distribution on the APD array(s). In this work, the various contributions to the energy and timing resolutions were analysed, as well as the influence of several contributions to the spatial resolution. Energy resolutions of around 11.5% FWHM at 511 keV were measured, the major part of which was found to be due to the intrinsic photon variance of the scintillator. A timing resolution of 1.6 ns FWHM was measured against a BaF₂-PMT detector. The timing resolution was found to be strongly gain dependent. Spatial resolutions of around 1.75 mm FWHM were measured, fairly independent of the APD gain. This figure still needs to be corrected for the ~0.9 mm FWHM measurement beam.

I. Introduction

IN most current positron emission tomography (PET) detector designs, based on small scintillation crystals coupled to position-sensitive photomultiplier tubes, sensitivity is lost due to the dead space between the crystals. Furthermore, the spatial resolution may be limited by inter-crystal scatter and parallax errors. Monolithic scintillation detectors, consisting of several cm³ of scintillating material coupled to one or more avalanche photodiode (APD) arrays, can avoid these problems. The entry point of an incoming annihilation photon on the detector front surface is derived from the distribution of the scintillation light on the APD arrays. This readout method, which does require a-priori knowledge of the incidence angle, intrinsically provides depth of interaction (DOI) information. Previous work has shown that detectors consisting of a LYSO crystal coupled to two APD arrays have a good spatial resolution, with very little degradation for angles of incidence up to 30° [1].